For example, according to the studies about a power converter such as a power supply by the present inventors, a power supply by a synchronous rectification method has been used so often with the achievement of low voltage driving of a power supply used for a CPU (Central Processing Unit) such as a computer. Also, the current slew rate (di/dt) required for the CPU power supply has been increasing more and more, and therefore, the development of a higher-speed power supply has been important for suppressing ripple of an output voltage of the power supply.
FIG. 1 shows a circuit diagram of a general non-isolated DC-DC converter for converting a direct-current voltage. An input capacitor Cin is arranged in parallel with an input power supply Vin. A drive capacitor Cdrive is inserted in parallel with a drive power supply Vdrive for driving a high-side switch Q1 and a low-side switch Q2, thereby supplying power to a low-side driver 33. A driver circuit for the high-side switch Q1 has a boot strap configuration, and when the high-side switch Q1 is turned off, current is supplied from the drive power supply Vdrive to a boot strap capacitor Cboot via a boot strap diode Dboot, thereby becoming a power supply for a high-side driver 32.
The high-side switch Q1 and the low-side switch Q2 are alternately turned on and off by receiving a PWM signal from a power supply controller 31, so that square waves are outputted to a terminal Vx and the square waves are smoothed to a direct-current voltage by an output filter configured with an output inductor L and an output capacitor Cout, thereby supplying power to a CPU 34 to be a load. Diodes D1 and D2 are connected in antiparallel to the high-side switch Q1 and the low-side switch Q2, respectively, and the diode D1 assumes a role of ensuring a current path to the input power supply Vin when a potential of the Vx to be one terminal of the output inductor L is increased. Also, the diode D2 assumes a role of ensuring a return current path of the output inductor L when the high-side switch Q1 is turned off. The synchronous rectification means an operation that the low-side switch Q2 is turned on to switch the current path from the diode D2 to the low-side switch Q2 when the return current flows in the diode D2, and this can reduce the conduction loss because on-resistance of the low-side switch Q2 is significantly low compared to the diode D2. When a MOSFET (Metal Oxide Semiconductor Field Effect Transistor) is used as the low-side switch Q2 as shown in FIG. 1, since a built-in diode is provided on the same semiconductor substrate, an external diode is unnecessary.
Conventionally, a discrete package power MOSFET is used as the high-side switch Q1 and the low-side switch Q2 of a non-isolated DC-DC converter, a driver IC including the high-side driver 32 and the low-side driver 33 for driving these switches is provided in a different package from the discrete package, and the switches are connected to the drivers on a printed board, respectively.
However, as the current slew rate (di/dt) of the power supply becomes larger, the reduction of conversion efficiency due to influences of a parasitic inductance on the printed board and a parasitic inductance caused by wire bonding in the package becomes nonnegligible.
Also, gate resistance and driver resistance exiting in the discrete package become the cause of decreasing the conversion efficiency of the DC-DC converter as well with the development of the high-speed power supply. For solving the problem, a monolithic structure in which the high-side switch Q1 and the low-side switch Q2, and the high-side driver 32 and the low-side driver 33 for driving these switches are mounted on the same semiconductor substrate has been proposed (for example, in Japanese Patent Application Laid-Open Publication No. 2005-203766: Patent Document 1). By mounting the high-side switch Q1 and the low-side switch Q2, and the high-side driver 32 and the low-side driver 33 for driving these switches on the same semiconductor substrate, the parasitic inductances can be reduced and the conversion efficiency of the power supply can be improved.